The unchanging face of RF

This year at NAB was somewhat unique in what wasn't there. In addition to 30,000 or so less people, there really weren't any huge, earthshakingly new items in the RF area. What was there was a continuing steady improvement in the technologies that have been emerging over the last several years.

Dielectric offers the FLEXLine coaxial cable, which is available in 7/8” through 6-1/8” line sizes and features patented precision-fitted connectors.

One item that, while seemingly small, was good to see, was the elimination of “crowbar” circuits in some of the UHF transmitters. As a refresher, the “crowbar” circuits were designed to dump all the existing energy in the power supply system when an arc was sensed in an IOT. This has always been something of a frantic circuit involving hugely wild currents. Now, manufacturers have developed switching power supply circuits that will remove the power source from an IOT quickly enough to save the device in a more calm and controlled manner.

This will be helpful with UPS systems. While these systems have worked capably with crowbar circuits, it has often been necessary to get the UPS manufacturer to adjust the equipment to a non-normal mode of operation. This was necessary in order to dump the UPS load back to the power line when the huge current demand hit during the crowbar activation. The new circuits will eliminate that sudden demand on the UPS.

Almost everyone now has controllers in their transmitters that permit access and adjustment in varying degrees from either a modem or via the Internet. Some have gone even further. As an example, Ai offers a service they call Remote Parameter Monitoring (RPM). In this system, the factory monitors a station's transmitter performance with trend-spotting software and notifies the station when conditions appear to indicate the development of a problem. This type of service has been offered by independent groups in the past. However, this differs from a “broad brush” approach in that it is offered by manufacturers only for the equipment that they have produced.

Liquid-cooled solid-state transmitters seem to be developing nicely. At least one brand of such transmitters allows the amplifier modules to be hot-swapped without dumping coolant all over the place. This makes the liquid-cooled systems as user-friendly as air-cooled units, while still gaining efficiency.

Speaking of efficient, some manufacturers now offer exciters that can be changed from analog to digital by simply changing a single circuit board or by software control. Essentially, a new transmitter can be purchased and used for analog until that great conversion in the sky takes place. At that time, the changeover can be done at the keyboard. Neat, huh?

Some exciting things have also occurred in the antenna and transmission line arena. First, who would have anticipated that the granddaddy of them all in rigid coaxial lines would introduce a line of semi-flexible cable? Dielectric, previously of rigid coaxial cable, truncated elliptical waveguide and rectangular waveguide fame, introducing a full range of semi-flexible cables. Dielectric has also improved the outer conductor attachment to the cable. One of the favorite games being played on the show floor was guessing at just who is making that line for Dielectric. As of this date, no one is talking.

As most readers are aware, semi-flexible waveguide became the transmission line du jour for most microwave systems many years ago. Those waveguides essentially eliminated large runs of rectangular microwave, as they were less expensive, easier to install and performed essentially as well. Their problem in the larger sizes has been the difficulty of consistent fabrication. They cannot be flexible, as that type of construction would not provide enough stability in the big sizes. Therefore, they must be rigid, making them somewhat more difficult to construct. In any case, Myat seems to have come up with just that, in their UHF rigid elliptical waveguide, subject to seeing the final tuning results. A brave soul was found and a full system was under construction at a station at the time of NAB. More complete information on the performance of a real-world version of the new waveguide should be available soon.

There were also some new items in the antennas area. Antennas are now available that work equally well on two channels — in the same or different bands — while avoiding the huge tower burden of either a major panel array or separate radiators in two different apertures.

The other area of new designs is what will be called inexpensive antennas. Not “cheap,” that would tend to indicate poor quality, which isn't the case. The quality of these antennas is fine, but they aren't capable of handling a lot of power and are usually rather limited regarding some of the niceties concerning beam tilt, null fill and widely varying patterns.

The less expensive antennas have always been popular in the translator or LPTV areas. The large demand from broadcasters seeking STA operations for DTV covering only the community of license requirement has motivated manufacturers to develop these inexpensive antennas. In many cases, they will find long-term use as auxiliary systems in case of failure in the main system.

In any case, the antennas will perform well. Their VSWR performance is at least very close to their more expensive relatives. As a rule, several standard patterns are available for directional operation. Most also have beam tilt available, although this sometimes has to be selected from “standard” values. The only big negative here is power handling capability and being able to tailor the pattern freely. It takes just as much engineering time to work out specialized patterns on the small antennas as for a larger, standard unit. Therefore, going after such modifications makes the price change rapidly. The place to use these antennas is where a fairly standard configuration will do the job and low power is acceptable. BE Don Markley is president of D.L. Markley and Associates, Peoria, IL.

Don Markley is president of D.L. Markley and Associates, Peoria, IL.

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